Introduction
Transfer of heat is a very common operation in connection with natural and human induced activities. Heat transfer mainly depends on three different mechanisms, namely conduction, convection and radiation.
Heat transfer by conduction is essentially characterized by no observable motion of matter. In metallic solids there is motion of unbound electrons and in liquids there is transport of momentum between molecules and in gases there are molecular diffusion (the random motion of molecules). Heat transfer by convection is essentially a macroscopic phenomenon that arises from the mixing of fluid elements, wherein natural convection may be caused by differences in density and forced convection may be caused by mechanical means. Heat transfer by radiation is essentially characterized by the presence of electromagnetic waves. All materials radiate thermal energy. When radiation falls on a second body it will be transmitted reflected or absorbed. Absorbed energy appears as heat in the body.
Transfer of heat in most heat exchangers takes place mainly by conduction and possibly convection as heat passes through one or several layers of material to reach a flow of heat absorbing fluid or gas. However, other transferring mechanisms may be involved to some extent. The layer or layers of material are normally of different thicknesses and with different thermal conductivities. Consequently, knowledge of the overall heat transfer coefficient is essential in the design of a heat exchanger. With known overall heat transfer coefficient the required heat transfer area is calculated by an integrated energy balance across the heat exchanger.
Heat exchangers are available in a number of various designs. The most common types are the tubular heat exchanger, the plate heat exchanger and the scraped surface heat exchanger. The choice of construction material differ depending on application. In the food industry the predominant materials are stainless or acid proof steel or even more exotic materials like titanium, the latter typically for fluids containing chlorides. In other industries heat exchangers made out of mild steel may be sufficient.
Plate heat exchangers are often used on low-viscous applications with moderate demands on operating temperatures and pressures, typically below 150° C. and 25 bars. Gasket material is chosen to withstand the operating temperature at hand and the constituents of the processing fluid. In the food industry plate heat exchangers are typically used for milk and juice pasteurisers operating at temperatures below 100° C. and pressures below 15 bars.
Tubular heat exchangers are typically used in applications where the demands on high temperatures and pressures are significant. Also, tubular heat exchangers are employed when the fluid contains particles that would block the channels of a plate heat exchanger. In the food industry tubular heat exchangers are typically used for milk and juice sterilisers operating at temperatures up to 150° C. Tubular heat exchangers are also used for moderate to high-viscous and particulate products, e.g. tomato salsa sauce, tomato paste and rice puddings. In some of these cases the operating pressure can exceed 100 bars. Particles up to 10-15 mm in size can be treated in tubular heat exchangers without problems.
Scraped surface heat exchangers are used in applications where the viscosity is very high, where big lumps are part of the fluid or where fouling problems are severe. In the food industry scraped surface heat exchangers are used e.g. on products like strawberry jam with whole strawberries present. The treatment in the heat exchanger is so gentle and the pressure drop so low that the berries will pass the system with only very little damage. The scraped surface heat exchangers is, however, the most expensive solution and is therefore used only when plate heat exchangers and tubular heat exchangers would not perform adequately.
Related Art
U.S. Pat. No. 5,251,603 (Watanabe et al.) discloses a fuel cooling system for a motor vehicle having; a fuel tank (2) for supplying fuel to a motor vehicle engine (E), a refrigerant evaporator (12), a compressor (8) of a refrigeration system for air conditioning and a heat exchanger (15) provided between a fuel pipe (3b) and an evaporated refrigerant pipe (13), see e.g. col. 2 lines 45-66 and FIG. 1. The heat exchanger (15) is made up of coaxial inner and outer tubes (17, 18) and, for example, helical heat transfer fins contained in a space between the inner and outer tubes (17, 18), see e.g. col. 3 lines 4-64 and FIG. 2-4. With this construction, the fuel flowing through a fuel return pipe (3b) extending between the engine (E) and the fuel tank (2) is caused to flow through the space between the inner and outer tubes (17, 18), whereas evaporated low temperature refrigerant is caused to flow through the inside of the inner tube (17) of the heat exchanger. The inner tube has secured therein, heat exchange fins, for example, of the type extending longitudinally thereof and having wavy transverse cross section. The fuel and the refrigerant exchange heat through the inner tube, whereby the fuel is cooled effectively.
U.S. Pat. No. 5,107,922 (So) discloses an offset strip fin (42) for use in compact automotive heat exchangers (30). The offset strip fin (42) has multiple transverse rows of corrugations extending in the axial direction, wherein the corrugations in adjacent rows overlap so that the oil boundary layer is continually re-started. The fin dimensions have been optimized in order to achieve superior ratio of heat transfer to pressure drop along the axial direction. In one aspect, a compact concentric tube heat exchanger (30) has an off-set strip fin (42) located in an annular fluid flow passageway located between a pair of concentric tubes (32, 34), see e.g. col. 5 line 44 to col. 7 line 6 and FIG. 1-4.
The heat exchangers disclosed in the above Watanabe and So are basically tubular heat exchangers. The exchangers in Watanabe and So are comparably small to fit in a limited inner space of a motor vehicle. The available heat transfer area is therefore limited, which demands a high temperature difference between two heat exchanging media to obtain a sufficient heat exchange. This is confirmed in Watanabe by the use of a compressor (8) for evaporating the refrigerant medium, which leads to a significant cooling of the refrigerant that flows through the inside of the inner tube (17).
WO 03/085344 (Jensen et al.) discloses a heat exchanger assembly comprising an inner tube (3) forming a first channel (24) for a first fluid and an outer tube (1) completely surrounding the inner tube (3) and extending in parallel with respect to the inner tube, which thereby defines a second channel (25) for a second fluid. Fins (2) are extending between the outside wall of the inner tube (3) and the inside wall of the outer tube (1). The fins (2) are integrated with the inner tube (3) only, see e.g. the abstract on page 1 and FIG. 1-2 in Jensen.
The heat exchanger in Jensen is basically a tubular heat exchanger. The heat transfer occurs through the wall and fins (2) of the inner tube (3). However, looking at the cross-section of the exchanger in FIG. 1-2 it can be seen that the wall and fins (2) of the inner tube (3) are comparable thick. The material in the wall and fins should therefore have a high thermal conductivity to provide a sufficient heat exchange. The thick fins (2) of the inner tube (3) will furthermore reduce the area that is available in the tube (3) for a heat transfer through the wall and fins of the inner tube (3). Typically, a reduced heat transfer area demands an increased temperature difference between the fluids to maintain a sufficient heat exchange. An alternative is to increase the pressure and/or flow of one medium or both media. This is especially so if a heat exchanger as the one in Jensen is used for a heat exchange between a gas medium and a fluid medium, or between two gas media. A gas medium has a lower density than a fluid medium and a gas medium is therefore typically not able to carry, receive or emit the same amount of energy per cubic unit as a fluid medium. This means that a heat transfer to or from a gas medium typically requires a larger heat transfer area compared to the area needed for transferring the same amount of energy to or from a fluid medium within the same time.
U.S. Pat. No. 5,735,342 (Nitta) discloses a heat exchanger system including an outer duct housing (20) and a powered fan (24) at one end. A heat exchanger including two nested pipes (28, 30) is positioned in line with the fan (24) within the duct (20). Each pipe (28, 30) includes radially outward fins (38, 46) and radially inward fins (40, 48). The radially inward fins (40) on the outer pipe (28) and the radially outward fins (46) on the inner pipe (30) are interleaved. End caps (32, 34) placed on the ends of the pipes include baffles (54, 56, 58, 68, 70), which appropriately divide annular manifolds (60, 62) defined between the pipes (28, 30) and between the ends of the fins (38, 40, 46, 48) and the end caps (32, 34) in order that four passes are possible through the length of the heat exchanger.
The inner pipe (30) defines an inner passage through the centre of the pipe (30). The radially inward fins (48) extend into that passage. The two end caps (32, 34) have holes (72, 74), which aligns with the passing trough the inner pipe (30). In this way, the fan (24) can force air through the interior of the heat exchanger as well as outwardly around the heat exchanger with flow in the longitudinal direction of the device, see col. 2 lines 58-65.
The heat exchanger in Nitta is similar to the heat exchanger in Jensen. However, the wall and the fins of the pipes in Nitta seem comparably thinner than their counterpart in Jensen. The demand for a high thermal conductivity in the material of the wall and fins may therefore be lower in Nitta. However, a substantial part of the cross-section in Nitta, as well as in Jensen, is occupied by the wall and fins of the inner pipe. This narrows the passage for the gas or the fluid or similar medium that is supposed to pass through the heat exchanger and the pressure of the medium may therefore have to be increased.
The prior art heat exchangers as described above display one or several of the following drawbacks; small heat exchanging area, high temperature differences, small cross-section for the flow of medium, high medium flow rate, high medium pressure.
The prior art heat exchangers are clearly unsuitable for exchanging heat between a slowly flowing gas medium and a flow of a fluid or liquid medium having a low temperature difference, and they are particularly unsuitable as heat exchangers for regulating the temperature of air slowly flowing through the exchanger for the purpose of regulating the temperature and air comfort in a defined space, preferably in an in door space.